Magnetic Nanoparticles: Synthesis, Anisotropy, and Applications.

Anisotropy is an important and widely present characteristic of materials that provides desired direction-dependent properties. In particular, the introduction of anisotropy into magnetic nanoparticles (MNPs) has become an effective method to obtain new characteristics and functions that are critical for many applications. In this review, we first discuss anisotropy-dependent ferromagnetic properties, ranging from intrinsic magnetocrystalline anisotropy to extrinsic shape and surface anisotropy, and their effects on the magnetic properties. We further summarize the syntheses of monodisperse MNPs with the desired control over the NP dimensions, shapes, compositions, and structures. These controlled syntheses of MNPs allow their magnetism to be finely tuned for many applications. We discuss the potential applications of these MNPs in biomedicine, magnetic recording, magnetotransport, permanent magnets, and catalysis.

Atomically Dispersed Cu Catalyst for Efficient Chemoselective Hydrogenation Reaction.

The Cu-based nanocatalysts have shown a high selectivity toward selective hydrogenation reaction, but the underlying catalytic mechanism is still murky. Herein, we report a new gram-scale strategy for realizing the single atom Cu site incorporated into the melem ring of graphitic carbon nitride (Cu1/CN) for understanding the catalytic mechanism of a hydrogenation reaction. The as-synthesized Cu1/CN exhibits unprecedented selectivity (100%), high activity (TOF = 2.9 × 103 h-1), and outstanding stability for selective hydrogenation of 4-nitrostyrene. We reveal that the presence of hydroxymethyl from trimethylolmelamine is beneficial to atomically disperse Cu atoms in the CN. X-ray absorption fine structure tests reveal that the Cu atom of Cu1/CN is dominated by the quaternary coordination way (Cu-N4) in the melem ring of CN. Density functional theory calculations confirm that the high reactivity and selectivity originate from the anchored Cu sites creating the optimal chemical environment for the highly efficient hydrogenation reaction.

A New Hexagonal Cobalt Nanosheet Catalyst for Selective CO2 Conversion to Ethanal.

We report a new form of catalyst based on ferromagnetic hexagonal-close-packed (hcp) Co nanosheets (NSs) for selective CO2RR to ethanal, CH3CHO. In all reduction potentials tested from -0.2 to -1.0 V (vs RHE) in 0.5 M KHCO3 solution, the reduction yields ethanal as a major product and ethanol/methanol as minor products. At -0.4 V, the Faradaic efficiency (FE) for ethanal reaches 60% with current densities of 5.1 mA cm-2 and mass activity of 3.4 A g-1 (total FE for ethanal/ethanol/methanol is 82%). Density functional theory (DFT) calculations suggest that this high CO2RR selectivity to ethanal on the hcp Co surface is attributed to the unique intralayer electron transfer, which not only promotes [OC-CO]* coupling but also suppresses the complete hydrogenation of the coupling intermediates to ethylene, leading to highly selective formation of CH3CHO.

Stabilizing Hard Magnetic SmCo5 Nanoparticles by N-Doped Graphitic Carbon Layer.

We report a chemical method to synthesize size-controllable SmCo5 nanoparticles (NPs) and to stabilize the NPs against air oxidation by coating a layer of N-doped graphitic carbon (NGC). First 10 nm CoO and 5 nm Sm2O3 NPs were synthesized and aggregated in reverse micelles of oleylamine to form SmCo-oxide NPs with a controlled size (110, 150, or 200 nm). The SmCo-O NPs were then coated with polydopamine and thermally annealed to form SmCo-O/NGC NPs, which were further embedded in CaO matrix and reduced with Ca at 850 °C to give SmCo5/NGC NPs of 80, 120, or 180 nm, respectively. The 10 nm NGC coating efficiently stabilized the SmCo5 NPs against air oxidation at room temperature or at 100 °C. The magnetization value of the 180 nm SmCo5/NGC NPs was stabilized at 86.1 emu/g 5 days after air exposure at room temperature and dropped only 1.7% 48 h after air exposure at 100 °C. The stable SmCo5/NGC NPs were aligned magnetically in an epoxy resin, showing a square-like hysteresis behavior with their Hc reaching 51.1 kOe at 150 K and 21.9 kOe at 330 K and their Mr stabilized at around 84.8 emu/g. Our study demonstrates a new strategy for synthesizing and stabilizing SmCo5 NPs for high-performance nanomagnet applications in a broad temperature range.

A Flame-Reaction Method for the Large-Scale Synthesis of High-Performance Smx Coy Nanomagnets.

We report a flame-reaction method to synthesize high-performance Smx Coy (x=1, y=5; x=2, y=17) particles on a multigram scale. This flame reaction allows the controlled decomposition of Sm(NO3 )3 and Co(NO3 )2 to 320 nm SmCo-O (SmCoO3 + Co3 O4 ) particles. A 5.8 g sample of SmCo3.8 -O particles was coated with CaO and then reduced at 900 °C by Ca to give 4.2 g of 260 nm SmCo5 particles. The SmCo5 particles are strongly ferromagnetic and the aligned particles in epoxy resin exhibit a large room-temperature coercivity (Hc ) of 41.8 kOe and giant (BH)max (maximum magnetic energy product) of 19.6 MGOe, the highest value ever reported for SmCo5 made by chemical methods. This synthesis can be extended to synthesize Sm2 Co17 particles, providing a general approach to scaling up the synthesis of high-performance Smx Coy nanomagnets for permanent magnet applications.

Changing Cinnamaldehyde Skeleton Achieves Antibacterial Nanoswitch.

Changeable substituent groups of organic molecules can provide an opportunity to clarify the antibacterial mechanism of organic molecules by tuning the electron cloud density of their skeleton. However, understanding the antibacterial mechanism of organic molecules is challenging. Herein, we reported a molecular view strategy for clarifying the antibacterial switch mechanism by tuning electron cloud density of cinnamaldehyde molecule skeleton. The cinnamaldehyde and its derivatives were self-assembled into nanosheets with excellent water solubility, respectively. The experimental results show that α-bromocinnamaldehyde (BCA) nanosheets exhibits unprecedented antibacterial activity, but there is no antibacterial activity for α-methylcinnamaldehyde nanosheets. Therefore, the BCA nanosheets and α-methylcinnamaldehyde nanosheets achieve an antibacterial switch. Theoretical calculations further confirmed that the electron-withdrawing substituent of the bromine atom leads to a lower electron cloud density of the aldehyde group than that of the electron-donor substituent of the methyl group at the α-position of the cinnamaldehyde skeleton, which is a key point in elucidating the antimicrobial switch mechanism. The excellent biocompatibility of BCA nanosheets was confirmed by CCK-8. The mouse wound infection model, H&E staining, and the crawling ability of drosophila larvae show that as-prepared BCA nanosheets are safe and promising for wound healing. This study provides a new strategy for the synthesis of low-cost organic nanomaterials with good biocompatibility. It is expected to expand the application of natural organic small molecule materials in antimicrobial agents.

The Electron Migration Polarization Boosting Electromagnetic Wave Absorption Based on Ce Atoms Modulated yolk@shell FexN@NGC.

The electron migration polarization is considered as a promising approach to optimize electromagnetic waves (EMW) dissipation. However, it is still difficult to realize well-controlled electron migration and elucidate the related EMW loss mechanisms for current researches. Herein, a novel FexN@NGC/Ce system to construct an effective electron migration model based on the electron leaps among the 4f/5d/6s orbitals of Ce ions is explored. In Fe4N@NGC/CeSA+Cs+NPs, Ce single-atoms (SA) mainly represent a +3 valence state, which can feed the electrons to Ce4+ of clusters (Cs) and CeO2 nanoparticles (NPs) through a conductive network under EMW, leading to the electron migration polarization. Such electron migration loss combined with excellent magnetic loss provided by Fe4N core, results in the optimal EMW attenuation performance with a minimum reflection loss exceeds -85.1 dB and a broadened absorption bandwidth up to 7.5 GHz at 1.5 mm. This study clarifies the in-depth relationship between electron migration polarization and EMW dissipation, providing profound insights into developing well-coordinated magnetic-dielectric nanocomposites for EMW absorption engineering.

Inverted All-Inorganic Nanorod-Based Light-Emitting Diodes via Electrophoretic Deposition.

High performance and high stability in all-inorganic solution processed nanocrystal-based light-emitting diodes (LEDs) are highly attractive for large area devices compared to organic material-based LEDs. In this work, an inverted all-inorganic LED structure is designed to have an easy integration with thin-film transistors. Adopting robust inorganic materials such as Ni1-x O nanoparticle films as a hole transport layer (HTL) is beneficial for the performance of LED. Herein, we have optimized the HTL by introducing Mg into Ni1-x O to bridge the difference in energy offset between the nanorod emissive layer and the HTL, in addition to the advantages of low temperature solubility of Ni1-x O:Mg nanoparticles. Furthermore, CdSe/CdS-based nanorods via electrophoretic deposition (EPD) are amassed in a vertically aligned (VA-NR) fashion as an emissive layer to facilitate the carrier transportation. Fostering these approaches enabled an EQE of 1.2% of the fabricated device, establishing the viability for further development of efficient and highly stable nanocrystal-based LEDs.

Highly Efficient Inverted Light-Emitting Diodes Based on Vertically Aligned CdSe/CdS Nanorod Layers Fabricated by Electrophoretic Deposition.

Inverted colloidal-nanocrystal-based LEDs (NC-LEDs) are highly interesting and invaluable for large-scale display technology and flexible electronics. Semiconductor nanorods (NRs), in addition to the tunable wavelengths of the emitted light (achieved, for example, by the variation of the NR diameter or the diameter of core in a core-shell configuration), also exhibit linearly polarized emission, a larger Stokes shift, faster radiative decay, and slower bleaching kinetics than quantum dots (QDs). Despite these advantages, it is difficult to achieve void-free active NR layers using simple spin-coating techniques. Herein, we employ electrophoretic deposition (EPD) to make closely packed, vertically aligned CdSe/CdS core/shell nanorods (NRs) as the emissive layer. Following an inverted architecture, the device fabricated yields an external quantum efficiency (EQE) of 6.3% and a maximum luminance of 4320 cd/m2 at 11 V. This good performance can be attributed to the vertically aligned NR layer, enhancing the charge transport by reducing the resistance of carrier passage, which is supported by our finite element simulations. To the best of our knowledge, this is the first time vertically aligned NR layers made by EPD have been reported for the fabrication of NC-LEDs and the device performance is one of the best for inverted red NR-LEDs. The findings presented in this work bring forth a simple and effective technique for making vertically aligned NRs, and the mechanism behind the NR-LED device with enhanced performance using these NRs is illustrated. This technique may prove useful to the development of a vast class of nanocrystal-based optoelectronics, including solar cells and laser devices.

A Self-Assembly of Single Layer of Co Nanorods to Reveal the Magnetostatic Interaction Mechanism.

In this work, we report a self-assembly method to fabricate a single layer of Co nanorods to study their magnetostatic interaction behavior. The Co nanorods with cambered and flat tips were synthesized by using a solvothermal route and an alcohol-thermal method, respectively. Both of them represent hard magnetic features. Co nanorods with cambered tips have an average diameter of 10 nm and length of 100 nm with coercivity of 6.4 kOe, and flat-tip nanorods with a 30 nm diameter and 100 nm length exhibit a coercivity of 4.9 kOe. They are further assembled on the surface of water in assistance of surfactants. The results demonstrate that the assembly type is dependent on the magnetic induction lines direction. For Co nanorods with flat tips, most of magnetic induction lines are parallel to the length direction, leading to an assembly that is tip to tip. For Co nanorods with cambered tips, they are prone to holding together side by side for their random magnetic induction lines. Under an applied field, the Co nanorods with flat tips can be further aligned into a single layer of Co nanorods. Our work gives a possible mechanism for the magnetic interaction of Co nanorods and provides a method to study their magnetic behavior.

Tip Interface Exchange-Coupling Based on "Bi-Anisotropic" Nanocomposites with Low Rare-Earth Content.

Specially designed SmCo5/Co magnetic nanocomposites have been fabricated by a "bottom up" process. SmCo5 nanochips were first prepared by solution-phase chemical synthesis combined with reductive annealing and then coated by chemical deposition of Co nanorods. Both the SmCo5 nanochips and Co nanorods are anisotropic and could be simultaneously aligned under the external magnetic field. Magnetic measurements applied on these "bi-anisotropic" SmCo5/Co composites show high magnetic performance with the Co phase content in a wide range from 10 to 80 wt %. For the first time ever, the applicable exchange-coupled nanocomposites with a rare-earth content lower than 7 wt % was realized, which exhibits the coercivity close to 10 kOe and remanence 31% larger than that of single phase SmCo5. 3-D micromagnetic simulations were performed to reveal that the reversal mechanism in the Co phase was transferred from the incoherent mode to the coherent mode under a tip interface exchange-coupling with a SmCo5 surface.

A unique synthesis of rare-earth-Co-based single crystal particles by "self-aligned" Co nano-arrays.

In this study, anisotropic SmCo5 magnets were prepared by a distinctive method, which is the high-temperature reductive annealing of Co@Sm2O3 with a specially designed nanostructure. High resolution transmission electron microscopy and elemental mapping show that the precursor self-assembly is composed of hcp-structured Co nano-rods with a coherent crystallographic orientation. During high temperature reduction, the Sm2O3 shell preserves the original morphology and alignment of these anisotropic Co nano-arrays, providing a template for hcp-structured SmCo5 single crystal particle synthesis. The as-prepared SmCo5 magnets exhibit well-controlled size and morphology, and a high coercivity of 30.9 kOe at room temperature. No stabilizer coating is necessary to prevent the formation of polycrystals in this synthesis.

Chemically synthesized anisotropic SmCo5 nanomagnets with a large energy product.

In this communication, we report a facile strategy to chemically synthesize anisotropic SmCo5 nanomagnets with a large magnetic energy product (BH). First, we designed a Co3O4@Sm2O3-CaO precursor by a one-pot method, which could be further reduced into uniform single-crystal SmCo5 particles under the stabilization of CaO coating. Following that, CaO was removed under an oxygen-free environment to impede oxidation. Finally, 130 ± 10 nm SmCo5 particles were aligned to be nanomagnet, exhibiting a large (BH)max value of 18.1 MGOe, which is the highest value reported by chemical methods.

Dispersible SmCo5 nanoparticles with huge coercivity.

It is difficult to obtain dispersed particles of SmCo5 by calciothermic reduction because of sintering during the high-temperature reaction. This study presents a new strategy to synthesize dispersible SmCo5 particles by co-precipitating a precursor containing amorphous Sm(OH)3 and coherent nanoscale Co(OH)2 and Ca(OH)2 crystallites. The Ca(OH)2 dehydrates into CaO which forms an isolation shell around the SmCo5 particles that prevents them sintering during the reaction at 860 °C. A magnetization of 90 Am2 kg-1, a remanence ratio of 0.96 and a huge coercivity of 6.6-7.2 T were achieved at room temperature after dissolving the CaO and orienting a dispersion of the particles in epoxy in a 0.8 T external field. Based on its scan-rate dependence in high quasi-static and pulsed magnetic fields, the coercivity mechanism is identified as nucleation and growth of 88 nm3 nucleation volumes in a low-anisotropy surface region about 15 nm thick. The coercivity is the highest yet reported for nanoparticles of any permanent magnet and it opens the prospect of new high-temperature magnet composites.

Designing shape anisotropic SmCo5 particles by chemical synthesis to reveal the morphological evolution mechanism.

In this work, we describe a new protocol to synthesize SmCo5 single crystal particles with remarkable shape anisotropy (hexagonal and rodlike), which exhibit a giant coercivity of 36.6 kOe and a high Mr/Ms value of 0.95 after an alignment. On this basis, the morphological evolution mechanism is illustrated by employing the template effect.

FePt/Co core/shell nanoparticle-based anisotropic nanocomposites and their exchange spring behavior.

Anisotropic exchange-coupled nanocomposites provide us a salient candidate for the new generation of permanent magnets owing to their huge predicted maximum energy product. However, previous research basically focused on thin films or bulk materials and the impact of easy-axis alignment on the exchange coupling behavior is not clear. Herein, strongly coupled FePt/Co core/shell nanoparticles with single-phase-like hysteresis loops were synthesized by the seed mediated method. Then, these nanoparticles were successfully aligned by the external magnetic field and fixed in an acrylic binder, so that FePt/Co core/shell nanoparticle-based anisotropic nanocomposites were obtained. The nanocomposites exhibited high degree of orientation as indicated by the increased remanence ratio from 0.62 for isotropic nanoparticles to 0.78 for anisotropic nanocomposites. However, a visible kink in the demagnetization curve was observed around the zero field, implying the exchange spring behavior. This result suggests that the aligned FePt cores impose a stronger overall dipolar field in Co shells and finally, force the Co shells to reverse at a low field before the switch of FePt cores. Our research extends the preparation methods of anisotropic hard/soft-phase nanocomposites and might be helpful for the design of high-performance anisotropic exchange-coupled nanocomposites.